The objectives of this proposal are to demonstrate the feasibility of an indirect flat-panel detector with programmable gain based on structured scintillator - cesium iodide (Csl), avalanche amorphous selenium (a-Se) photoconductor (which we call HARP) with increased gain and active matrix (AM) thin film transistor (TFT) readout, and improve the low dose imaging performance of flat-panel x-ray imaging detectors. We call the detector under investigation SHARP-AMFPI (Scintillator HARP Active Matrix Flat Panel Imager). It will provide better image quality in fluoroscopy, behind dense breasts in mammography, and digital tomosynthesis, where existing flat-panel detectors fall short of quantum noise limitation. Furthermore the inherent high electric field and charge carrier mobility in HARP allows better temporal imaging performance which leads to less image artifacts and better diagnosis accuracy. The specific aims are to: (1) understand and optimize the fundamental imaging properties of Csl and HARP for different x-ray imaging applications; (2) demonstrate the excellent low dose and high speed performance of SHARP using a prototype HARP tube optically coupled to optimized Csl layers; (3) identity potential practical difficulties for constructing SHARP-AMFPI and develop engineering methods for solving them; (4) demonstrate the practical feasibility of the proposed detector. Our research design and methods are to develop experimental and modeling techniques to investigate the fundamental imaging properties of Csl and HARP, verify the validity of models with experimental results so that the model can be used to optimize detector design parameters. We will investigate the feasibility of the detector using three gradual steps: First, the advantage of avalanche gain on image quality will be demonstrated using the matured electron beam readout technology for HARP, then a hybrid prototype detector will be made based on existing TFT array and HARP layer deposition technology so that compatibility issues between these two devices can be fully investigated, and finally a integrated prototype detector will be constructed using procedure that is scalable to large areas to demonstrate the ultimate feasibility of SHARP-AMFPI. Its x-ray imaging performance will be evaluated and compared to existing FPI technologies, so that its improved performance at low dose and high frame rates can be demonstrated.